var background = function(){
	var can = document.getElementById("student");
	//设置2d绘图环境
	var ctx = can.getContext("2d");

	//获取浏览器窗口的宽高
	var w = can.width = window.innerWidth,
		h = can.height = window.innerHeight;
	//自适应浏览器窗口
	window.onresize = function () {
			w = can.width = window.innerWidth,
			h = can.height = window.innerHeight;
	};
	//         ctx.fillStyle="yellow"
	//       ctx.fillRect(100,100,100,100);
	//       //  绘制圆形
	//    ctx.arc(250,250,50,0,Math.PI*2,false);
	//    ctx.strokeStyle="yellow";
	//    ctx.stroke();
	// //运动
	// var y=0;
	// setInterval(function(){
	// y++;
	// ctx.clearRect(0,0,w,h);
	// ctx.fillRect(100,y,100,100);
	// },30);
	function Drop() {}; //创建雨滴类
	Drop.prototype = {
		init: function () {
			this.x = rand(0, w); //雨滴的初始化坐标
			this.y = 0; //雨滴y轴方向的坐标
			this.vy = rand(8, 9); //雨滴下落的速度
			this.l = rand(h * 0.8, h * 0.9); //雨滴下落的高度
			this.r = 1;
			this.vr = 1; //半径增加的速度
			this.a = 1;
			this.va = 0.98; //透明度的变化系数
		},
		draw: function () //绘制雨滴
		{
			if (this.y > this.l) {
				//绘制圆形
				ctx.beginPath(); //开始路径
				ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2, false);
				ctx.strokeStyle = "rgba(255,255,255," + this.a + ")";
				ctx.stroke();

			} else {
				//绘制下落的雨滴
				ctx.fillStyle = "rgb(255,255,255)";
				ctx.fillRect(this.x, this.y, 2, 10);
			}
			this.update();
		},
		update: function () //更新坐标
		{
			if (this.y < this.l) {
				this.y += this.vy;
			} else {
				if (this.a > 0.03) {
					this.r += this.vr;
					if (this.r > 50) {
						this.a *= this.va;
					}
				} else {
					//重新初始化了
					this.init();
				}
			}
		}
	}
	//实例化一个雨滴对象
	var drops = []; //默认值为undefined
	//console.log(drops)
	for (var i = 0; i < 30; i++) {
		setTimeout(function () {
			var drop = new Drop();
			drop.init();
			drops.push(drop);
		}, i * 200)
	}
	//实例初始化
	setInterval(function () {
		//绘制一个透明层
		ctx.fillStyle = "rgba(56,163,179,0.1)";
		ctx.fillRect(0, 0, w, h);
		for (var i = 0; i < drops.length; i++) {
			drops[i].draw();
		}
	}, 30);

	function rand(min, max) {
		return Math.random() * (max - min) + min;
	}
}
export default background ;
